1. Technical Field
The present invention relates to a concentration measurement apparatus and a concentration measurement method that quantitates a concentration of target component among an observation target composed of a plurality of layers of light scattering medium, in a non-invasive manner and with an excellent accuracy.
2. Related Art
In recent years, Japan is said to be in the era of satiation, with yearly increasing number of diabetes patients. The number of diabetic nephropathy is also increasing, resulting in an yearly increase of ten thousand patients of chronic renal failure, with a total patient number of over 280 thousand.
On the other hand, under increasing demand of preventive medicine in the advent of the aging society, the importance of the personal metabolic rate control is drastically rising. In particular, it has been known that by blood sugar level measurements before and after meals, the sugar metabolism reaction can be evaluated. Evaluation of the sugar metabolism reaction in the very early stage of diabetes enables an early treatment of diabetes based on early stage diagnosis.
In the past, blood sugar level measurement has been done by taking blood samples from veins in the arm or the finger tip, and measuring enzymatic activity to the glucose in the blood sample. However, there have been various problems in that such blood sugar level measurement requires tedious and painful blood drawing and also involves risk of infectious disease.
Moreover, as a method to continuously measure blood sugar level, devices have been developed in the united states, that continuously perform glucose quantitation corresponding to the blood sugar level while an injection needle is kept inserted. Such devices are currently under clinical trials. However, keeping the injection needle inserted in the vein causes risks of unintended removal of the needle during the blood sugar level measurement, and also risks of infectious diseases.
Under these circumstances, demands has been growing for a development of blood sugar level measurement apparatus that can measure blood sugar level frequently without blood drawing, and is free of infectious disease risks. Demands also has been growing for a development of blood sugar level measurement apparatus that is easily and continuously wearable, and capable of being downsized.
As apparatus that non-invasively measures component concentrations, proposed devices adopt regular absorptiometric principals and use the methods of molecular absorbance measurement.
Such apparatus irradiates light with a specific wavelength or continuous wavelength on the measurement subject, measures the light absorbance, and calculates the component concentration based on the Beer-Lambert law.
However, apparatus that calculates glucose concentration based on the Beer-Lambert law has a problem in that under circumstances where the subject components interact with each other, e.g., cluster formation, in which the Beer-Lambert law is not applicable, a precise measurement can not be performed. For example, when the glucose level in the blood is measured, because of the influence of the interaction between glucose and water (liquid) in the blood, and the interaction between water and salt in the blood, precise measurements of concentration of such components are difficult.
On the other hand, some apparatus acts without relying on the Beer-Lambert law, in which a calibration curve is prepared in advance using subject materials with known concentrations, and then a sample material with unknown concentration is subject to absorbance measurement, and the absorbance is compared with the calibration curve to obtain the concentration of the subject sample material (see, e.g., JP-A-52-63397 and JP-B-3903147).
However, such measurement apparatus using the calibration curve, has a problem in that when an unexpected component other than the subject component is included, the variation in concentration of the unexpected component causes a change in the absorption by the subject component, resulting in a considerable measurement errors.
Some other apparatus using the calibration curve takes into account this concentration variation of components other than the subject component, and utilize a multivariate analysis (see, e.g., JP-A-2003-050200 and JP-A-2007-259967).
In such measurement apparatus (measurement methods) using the multivariate analysis, the calibration curve is prepared using a simulative method, in which the interaction between the components are not considered. In cases including multiple subject components, those components interact with each other. As the interaction between the components becomes larger, the measurement error of the concentration also becomes larger, causing difficulties in accurate measurements of the subject components.
It is conceivable to prepare a calibration curve by, instead of simulations, actually measuring multiple samples. However, such calibration curve preparation in consideration of mutual interactions is not practical because it excessively consumes time and work.